Structural and functional shifts of mercury methylating microorganisms during an exceptional drought can elevate ecosystem risk of nearshore area.

Mercury (Hg) is a global pollutant and its transformation to the highly toxic methylmercury (MeHg) is mediated by anaerobic microbes carrying hgcA gene, which can affect human health risk through fish consumption. The Pearl River Estuary in southern China is productive but suffers from increasing anthropogenic stresses. However, it remains poorly known regarding the environmental response of hgcA+ microorganisms to climatic variations. We demonstrated that saltwater intrusion driven by prolonged drought led to elevated sedimentary MeHg concentrations, particularly in the nearshore zone, compared to a wet year. Drought-induced saltwater intrusion served a dual microbial function: it reinforced a non-random association between oligotrophic Acidobacteriota (putative methylators) and Myxococcota (putative non-methylators), while simultaneously promoting a distinct, copiotrophic microbial module adapted to warmer, saline conditions. The significant correlation of module phyla with MeHg levels, coupled with its genetic capacity for sulfate reduction, points to a potential mechanism for the enhanced Hg methylation. Thus, climate-anthropogenic interactions can regulate MeHg risk by altered microbial community structure and interactions in estuarine ecosystems.